Monitoring railroad track safety and performance by sensing the Rail Neutral Temperature (RNT) or Stress Free Temperature (SFT) is known in the art. U.S. Pat. No. 7,502,670 entitled “System and Method for Determining Rail Safety Limits, for example, discusses methods of monitoring the RNT at a single point over a long period of time to determine changes in track geometry and strength. U.S. Pat. No. 7,502, 670 and the related U.S. Pat. No. 7,869,909 are incorporated by reference.
FIG. 1 illustrates the concept of RNT and SFT over a range of temperatures, along the x axis, and mechanical stress, along the y axis, in graph 100. In this description, both rail stress and temperature are dimensioned in degrees Celsius. The RNT 115 is defined as the rail temperature at which the rail experiences zero stress. Rail temperatures above the RNT tend to create compression in a constrained rail. Rail temperatures below the RNT tend to create tension in a constrained rail.
Railroad rail incurs tensile and compressive stresses as it heats and cools under ambient temperature conditions. The rail typically behaves in a range between two limiting cases when installed in track. Limiting Case A 105 is the fully constrained case wherein all thermal energy absorbed or dissipated by the rail produces stresses proportional to temperature. On the ordinate, the units of stress are expressed in temperature degrees, which provides a −1 slope in the fully constrained case.
Limiting Case B 110 describes a rail that is completely free to expand/contract with temperature changes. Case B 110 is not normally observed in functional track except for very limited circumstances near bolted rail joints and/or special slip joints. In practice, Case A 105 most closely approximates the observed case with some small differences due to less than perfect constraints in the track structure. Each point on the two curves is determined from a point pair of stress and temperature.
Rail failure due to tensile longitudinal rail loads or track panel buckling due to compressive longitudinal rail loads have a common characteristic, i.e. with a greater magnitude of the stress and forces comes a higher the probability of a track failure. In either tension or compression, the probability is approximately linear. The probability of failure is also relatively low when below the design limits of the track. However, when the loads are some significant percentage of the design limits, then flaws in the rail, excessive impacts by rolling stock, or less than intended support conditions in the crossties/ballast may produce an unexpected failure.
Whether above or below design limits, a common circumstance occurs in every case where there is a failure. Mechanical energy is released from the rail system, causing the calculated RNT to collapse towards the stress free state. For failures in tension, e.g. rail breaks, as exemplified by arrow 125, tensile stresses are removed. For failures in compression, e.g. buckling failures as exemplified by arrow 120, compressive stresses are removed.
In addition, referring briefly to FIG. 3, the closer the point of measurement is to the point of failure, the closer the post failure state will be the true (new) RNT. The FIG. 2 graph 200 illustrates the situation where a tensile rail stress failure is measured at a distance from the break location. Case C 205 illustrates a typical observed RNT curve in which a release of stress due to a track failure essentially shifts the Case A 105 down by the amount of stress release at 210, and where the new RNT 215 is lower than the pre-failed RNT 115. It can easily be observed in FIG. 2 that the opposite case can be true: If Case C 205 results in a compressive failure, energy is released by buckling and a reduction in compressive stresses occurs at 220. The RNT 215 in this case would increase to RNT 115.
In the case of a tensile failure of the track very near the point of measurement, the vertical drop is to the zero stress Case B curve. In the case of a complete buckling failure, the ‘jump’ up to the zero stress state to the Case B curve occurs.
In either of the failure cases, the data point pairs will be clearly different than any prior history that may have been collected. Therefore such a failure can only be predicted by monitoring both of temperatures and stresses.
Both of the above-described patents utilize the above-described concepts in a system and method that issues an alert if a long-term change in RNT is of sufficient magnitude. The prior art system and method is forced to rely on long-term changes in RNT because the system is susceptible to noise and elastic rail behavior, e.g. periodic geometric cycling of the rail due to temperature and elastic movement on the rail bed. These factors prevent the prior art systems and methods from identifying rail track faults in real time, e.g. shortly after a rail failure has occurred. Such a system is needed in order to improve rail safety, reduce the costs of maintenance, and to reduce failure-related downtime.